Maybe some glute stretching might be a good idea.
The classic is sitting on a good chair, feet flat on the floor, back straight - bring one leg up and put the ankle on the top of the other knee. Now for the next bit, it's important to keep the lower back straight - think of it as pivoting forwards from the hips NOT bending the back.
Put your hands on the knee and foot of the top leg and slowly and carefully (especially if you're not flexible, just getting one ankle onto the other knee will probably give you a stretch in the glutes) lean forwards as if trying to put your chest on the calf of the top leg. You should feel a stretch localised strongly at the bottom of your glutes - where the hamstrings insert basically. Hold the stretch for a count of 20 and then slowly sit up. Remember about the back straight - if you let the back bend, all you'll be doing is a back bend. Keeping the back straight forces the pelvis to pull against the hamstrings and glutes - hence the stretch.
The same exercise can be done on the floor - lie on your back and bring one leg up until the shin is horizontal, cross the other leg over it until the ankle's on the first knee - just as in the chair. Then put your hands around the first leg's thigh and pull gently towards your chest. With all your weight on your back, it's easier to keep your back straight. You can also vary the position of the 'cross' - if you cross the foot of the second leg onto the knee of the first, you'll get a different stretch from either the ankle, or if you can do it, the shin, as you're changing the angle of the hip. By moving from foot to ankle to shin you're moving the stretch from the centre of the glutes to the outside so more focus on the glueus medius - which is good for runners and ITB problems.
